1. Field of the Invention
The object of the invention is to compensate the temperature drift of a detector and a control signal, in other words to compensate the offset caused by the temperature change, in a periodic control action in order to reduce the variation caused to the controlled phenomenon, particularly in a mobile station such as a mobile phone.
2. Description of Related Art
Such periodic control actions are at least the Automatic Gain Control (AGC) and the transmission power control. Such a controlled phenomenon is generally the power of an RF (radio frequency) signal. At the control moment the control signal sets the desired control result, such as the power of the signal to be amplified. The temperature drift of the detector is at least partly due to a change in the threshold voltage of the diode, and the temperature drift of the control signal is at least partly due to a change in the output voltage of the digital-to-analogue converter. Further errors can be caused by a changed dependence between the inputs of the differential amplifier.
A problem in low-power applications is particularly the temperature drift on one hand of the detector diode in a detector indicating the magnitude of the power, such as a diode detector, and on the other hand of the power control signal. For instance, the temperature drift of a simple diode detector is typically 2 mV/xc2x0 C. The detector receives a signal with an amplitude, which as a minimum can be only a few millivolts. When the width of the temperature range is for instance 100xc2x0 C. it causes temperature drift which is many times higher than a signal with a small amplitude. In order to reduce the temperature drift it is known to compensate a diode detector according to FIG. 1A with a second diode. The compensation is made in the block B1 by a circuitry after the capacitor C1 which connects the RF signal. The circuitry includes an ordinary pull-up resistor R1C and a pull-down resistor R2C, a compensating diode D1C connected between the resistors in the forward direction, and a capacitor C1C connected in parallel with the compensating diode D1C and the pull-down resistor R2C, whereby the signal line is connected through the resistor R3C to the operating voltage side of the compensating diode. The temperature changes which act on the compensating diode D1C are the same or almost the same as those which act on the detector diode D1 which connects the positive half-periods of the RF signal supplied by the capacitor C1 to the integrating capacitor C2 and the load resistor R3. The circuitry is designed so that changes in the characteristics of the compensating diode D1 and the detector diode D1 will cancel each other, so that no temperature drift will occur. The voltage VDET supplied to the terminal RFIN which corresponds to the power of the RF signal is obtained at the integrating capacitor C2 and the load resistor R3.
The circuitry according to FIG. 1A reduces the temperature drift, but the currents with different magnitudes passing through the compensating diode D1C and the detector diode D1 will not enable a complete compensation of the temperature drift. Further the realisation of this circuitry requires many components.
There is also a known detector of periodic power according to FIG. 1B which prevents temperature drift and has a power detector B2 based on a diode D1, an amplifier B3, and a sampling circuit B4. The power detector B2 operates as follows: an RF signal is connected to the input RFIN and further to the capacitor C1, from where the transferred signal acts, via the connection point of the resistance bridge R1, R2 between the operating voltage and the ground and through the diode D1, on the capacitor C2 which can be charged with a positive potential and on a corresponding discharging resistor R3, whereby a voltage level corresponding to the power is supplied to the switch S1. The amplifier B3 operates as follows: in the OFF position the switch S1 switches the reference voltage to the capacitor C3 when no RF signal is transmitted or supplied to the periodic power detector; during signal reception the switch S1 is connected to the ON position, whereby the signal is connected via the resistor R4 to the amplifier A1 having a feedback via the resistor R5. The output of the amplifier A1 provides at the point OUT1 a power detection result which is proportional to the difference of the idle state power and the transmitted power, and this detection result is further supplied to the sampling circuit B4. The sampling circuit B4 operates as follows: at the sampling moment the switch S2 directs the output voltage OUT1 of the amplifier A1 to the capacitor C4, and the voltage of the capacitor is repeated by the amplifier A2 to the point OUT2. A power detector of this kind is used to correct the control of the transmit power of the transmitter.
The circuit according to FIG. 1B will effectively reduce the temperature drift.
A problem in known detectors and controllers is the mutually independent temperature drifts of on one hand the output signal of the detectors used to generate the gain control voltage and on the other hand the control signal setting the magnitude of the gain. The circuitries presented above reduce the temperature drift of the power detector. Typically the control signal is generated by a digital-to-analogue converter having a temperature drift corresponding to the residual temperature drift of the compensated diode detector according to FIG. 1A.
The object of the invention is to present a control based on a detector and a control signal, so that the control compensates the drift of the output signal of the detector and of the control signal caused for example by temperature variations.
The invention relates to a method for reducing the effects of drift of a detector of transmitted power and of a transmission power control signal in a transmitter arranged to transmit in an intermittent fashion, the output signal from the detector and the control signal controlling at least in part the transmission power of the transmitter. According to the invention, the method comprises at least the steps of storing of a voltage difference between the output voltage of the detector and the control signal during a pause in transmission, and using said stored voltage difference for compensating the drift of the detector and the control signal during a transmission period. The invention also relates to a system for compensating drifts in a transmitter arranged to transmit in an intermittent fashion, the transmitter having at least a detector of transmitted power and a control signal line for controlling the transmitted power. According to the invention the system comprises at least means for storing the voltage difference between the output voltage of the detector and the control voltage in the control signal line during a pause in transmission, and means for compensating the drift of the detector and the control signal during a transmission period using a stored voltage difference stored by said means for storing.
According to a further advantageous embodiment of the invention the system further comprises a compensation and control block for the compensation of the temperature drift of the detection and of the control signal. In a still further embodiment of the invention the compensation and control block further comprises the compensation of the temperature drift of the differential amplifier.
According to a further advantageous embodiment of the invention a system for reducing the effect of the temperature drift of a detector and of a control signal and for compensating the effect of the bias voltage variation in a diode detector is provided, which system comprises a diode detector for detecting the transmit power of an RF time division signal. According to the invention the effect of the bias voltage variations of the diode detector is compensated in the transmit power diode detector, and it comprises
a detector diode which is connected so that its forward direction is towards the switching means,
a bias resistor connected to a bias voltage, whereby the effect of the variations of the bias voltage is being compensated,
an integrating capacitor for equalising the voltage proportional to the RF power, and
a series resistance for connecting the voltage further via a resistance division, and the arrangement comprises further a compensation and control block to compensate the temperature drift of the detector and of the control signal, which arrangement comprises
an impedance from a constant potential to a connection point,
a feedback impedance,
a differential amplifier,
a control signal connection resistance for adding the control and the diode detector""s signal to the positive input of the differential amplifier,
a compensating capacitor between the connection point and the negative input of the differential amplifier,
a compensation switch between the inputs of the differential amplifier for arranging the charge of the compensating capacitor for the compensation by closing the compensating switch with a switch control signal during the pause.
According to a still further aspect of the invention, a mobile communication means is provided. According to this aspect of the invention, the mobile communication means comprises a system for compensating drifts in the transmitter of the mobile communication means, and in said system, means for storing the voltage difference between the output voltage of the detector and the control voltage in the control signal line during a pause in transmission, and in said system, means for compensating the drift of the detector and the control signal during a transmission period using a stored voltage difference stored by said means for storing.
According to the invention the transmission pause in the time division radio system is used to compensate the drifts of the control of the transmit power caused for example by temperature variations. For instance, in the GSM system the length of the transmission burst is about 600 xcexcs, and the lime ratio of the transmission state/pause is xe2x85x9. The compensation is based on a low or almost negligible change of the temperature during the transmission, and thus on a possibility to perform the compensation during a transmission pause. During the transmission pause there is no transmit power, so the output of the detector goes to a dc voltage corresponding to the state without any signal. Then the control signal is also a constant voltage. In this situation there is not allowed any voltage difference between the inputs of the differential amplifier, and the output voltage must be zero or a constant voltage which is lower than the level which causes the operation of the output stage, so that the power control performed at the beginning of the transmission state could be made error-free. The power is increased by increasing the voltage of the control signal to a level corresponding to the desired power.
The temperature drift of the detector and of the control signal has an impact on the power during the transmission pause which should be zero, on the power increase, and on the transmit power during the transmission. However, the transmit power can be prevented during a transmission pause, because the operation of the differential amplifier and/or the power amplifier generally also is switched on or off with the aid of a particular control signal. Then the error caused by the temperature drift will appear as a discontinuity in the power control between the transmission state and the pause, and it will not be possible to perform a controlled power increase or decrease. When the temperature drift is large it causes also a detectable magnitude error in the increased transmit power.
The transmission pause is used for the compensation of the transmit power by switching, during the transmission pause, the inputs of the differential amplifier to each other and by connecting the output to a constant potential, such a s the ground, whereby the capacitor connected, in a manner according to the invention between on one hand the connection point of the detector and differential amplifier""s feedback circuit and on the other hand the output of the differential amplifier, is charged to a value corresponding to the error caused by the temperature drift of both the detector and the control signal, when the second input of the differential amplifier is connected to the control signal. At the end of the transmission pause the circuitries used for the compensation are released, and the power controller is in balance so that despite the temperature drifts of the detector and the control signal there is no voltage between the inputs of the differential amplifier, and the output of the differential amplifier has no voltage compared to the ground potential. When the transmission state begins the voltage of the control signal is increased and both the gain control voltage and the transmit power will increase in a controlled manner. Due to the high gain of the differential amplifier and the power amplifier and due to the slowness of the power detection and the power amplifier control, the differential amplifier most preferably has a feedback and its output is low-pass filtered in order to maintain a stable control. However, the feedback and the low-pass filter and the required other components are prior art, and they can be designed on the basis of the characteristics of the power amplifier and the differential amplifier.
Advantageous embodiment of the invention are presented in the dependent claims.